1. Field of the Invention
The present invention relates to a servo control apparatus for controlling an optical pickup in an optical disc reproducing apparatus.
2. Description of the Related Art
A servomechanism which performs servo-control such as tracking-servo and focus-servo control is generally provided in a reproducing apparatus for reproducing an optical disc, for example, a CD (Compact Disc) or a DVD (Digital Versatile Disc). Reading position of the optical pickup is controlled by the servomechanism to attain an excellent reproduction performance. The optical disc usually has various defects including surface scratches, flaws, dirt or stains which are formed when the disc is manufactured, used or handled. The defects, however, impairs the stable operation of the servomechanism.
FIG. 1 is a sectional view of an optical disc 1, showing major defects occurring on the optical disc 1. The optical disc 1 has a protection layer 3, a reflection layer 4 for reflecting an optical beam emitted from an optical pickup, and a transparent cover layer 5 formed of a transparent material such as plastic.
In FIG. 1, defect I is a flaw, called an xe2x80x9cinterruptionxe2x80x9d, formed on the reflection layer 4 during manufacture of the disc. Defect II is dirt, called xe2x80x9cblack dotsxe2x80x9d, deposited on a disc surface, or the transparent cover layer 5. Defect II is a fingerprint or the like of human fat left on the disc surface 7, and is called a xe2x80x9cfingerprintxe2x80x9d. Defect IV is a flaw, called a xe2x80x9cscratchxe2x80x9d, left on the disc surface 7.
There causes a momentary loss of a read signal (RF signal) generated in the optical pickup, which is called a xe2x80x9cdropoutxe2x80x9d, when the above-described defects exist on the disc. This incurs performance degradation of servo control including tracking control, focus control and tilt control. Conventionally, the method has been employed, as a method for avoiding adverse effect upon servo control caused by such defects, that servo control is continued when detecting a defect by holding a servo-control amount (i.e., a tracking error value, a focus error value, etc.) in a value prior to the defect detection (hereinafter, simply referred to as xe2x80x9cpre-value holdxe2x80x9d).
However, the form or shape of the dropout caused in the RF signal generated in the optical pickup differs depending on the kind or type of the foregoing defects. Therefore, it is not always preferable to use the pre-value-hold method to continue servo control for every kind of the defects. There is a problem that some kinds of defects bring about worsening in control performance or deterioration in reproduction quality.
The present invention has been made from the above-described viewpoint, and it is an object of the present invention to provide a servo control apparatus capable of carrying out servo control with accuracy regardless of the kind of the defects.
To achieve the object, according to the present invention, there is provided a servo control apparatus of an optical pickup for reading recorded information from a recording medium and generates a read signal, which comprises an error signal extracting part for extracting an error signal which indicates a deviation amount of a reading position from a target reading position of the optical pickup; a drive part for driving the optical pickup to change the reading position; an equalizer for equalizing the error signal and supplying a drive signal which indicates a driving amount of the drive part; a detecting part for detecting a dropout portion in the read signal; an identifying part for identifying a form of the dropout portion detected in the detecting part; and a controller for changing a gain of the equalizer in accordance with the form of the dropout portion identified by the identifying part.